Environmental contamination due to high emission levels of greenhouse gases like carbon dioxide (CO2) continues to escalate. Sources include industrial fuel processes and fossil fuel burning, which remain the largest contributors despite implementations of gas capture technologies to remove CO2. The quandary is also reaching levels that are more complex as the world energy crisis has called the attention of many scientists and engineers to develop alternatives to petroleum based fuels. One of these options includes increasing natural gas production, but the presence of considerable amounts of nitrogen (N2) and CO2 in the natural gas effluents has raised questions regarding energy density when compared to other alternatives. The latter contaminant could be removed via chemical adsorption, pressure swing adsorption, membrane separation and cryogenic distillation, but these strategies are either energy intensive or involve expensive operational processes.
The challenges associated to CO2 removal from gas mixtures are not exclusive to energy-related applications. With the prospective space exploration long-term missions, the need for better CO2 ultra-purification systems is going to be a critical aspect. Although the breathing air that ought to be supplied to the spacecraft cabins will require ultra-low CO2 concentrations, the most important challenge is to find a process that provides uninterrupted air purification with minimal onboard energy and work impact. One possible solution to all of the aforementioned problems is the use of enhanced adsorption processes, including development of sorbent materials with pore-window tailoring capabilities, large pore volume and with outstanding CO2 selectivity under physisorption level interactions. The last criterion is critical for ease of regeneration, especially in space related applications.
For many years, Zeolite-like materials have been considered for CO2 removal from gas mixtures. One of these materials include one synthesized by Kuznicki and co-workers during the 1990s called Engelhard Titanium Silicate variant number 4 (ETS-4), which was obtained by using hydrothermal synthesis and exhibited a pore size between 3 and 4 angstroms. Several studies have found that the structure of ETS-4 is similar to the one of exhibited by the Zorite mineral, the key difference being the chemical composition. This material contains tetrahedral coordinated silicon and both square-pyramidal/octahedrally coordinated titanium framework atoms and was the first molecular sieve ever to exhibit such atomic arrangement. Depending on the activation (degas) temperature, a structural contraction or shrinkage is obtained. The final pore dimensions depend on the amount of structural water that has been removed from the sorbent material. It should be mentioned that this material has been used for CO2 adsorption from natural gas mixtures, and it has been reported that it has a substantial selectivity of CO2 over CH4, N2, O2 and Ar.
The adsorbed amounts in ETS-4 could be further increased by employing ion exchange methods, where extra-framework cations (monovalent or divalent) are incorporated and serve as unique adsorption sites. Cations also play an important role in the material phase when employed in hydrothermal synthesis since they are considered an important factor in the final crystallinity, morphology and yield. For instance, sodium acts as a structure-directing agent in the synthesis and provides effective adsorption sites that could be modified to alter the framework properties.
Although the sorption and flexible features of ETS-4 are extraordinary, the available pore volume requires additional cycles during pressure swing adsorption applications. Thus, the present invention provides a novel solution to this problem, by presenting a new titanosilicate called University of Puerto Rico at Mayaguez variant number 5 (UPRM-5) prepared using Tetraethylammonium Hydroxide (TEAOH) as a molecular structure-directing agent (SDA). Although the use of TEAOH is known to have a considerable effect on the nucleation, crystallization and void volume of materials, there are no reports on its use to prepare flexible titanium silicates. In addition, the invention presents a non-destructive detemplation process and two sorbent variants based on strontium (Sr2+) and barium (Ba2+) cations. UPRM-5 and the ion-exchanged variants have been characterized here via powder X-ray diffraction (XRD), Fourier transform infra-red (FT-IR) spectroscopy, thermal gravimetric analysis (TGA), scanning electron microscopy (SEM) and porosimetry techniques, all aiming at elucidating the structural properties relevant to the material adsorption performance. Adsorption isotherms for CO2, CH4, N2 and O2 on all the strontium- and barium-based sorbent materials as well as CO2 isosteric heat of adsorption on the Sr2+ and Ba2+ variants are also presented.
Throughout the figures, the same reference numbers and characters, unless otherwise stated, are used to denote like elements, components, portions or features of the illustrated embodiments. The subject invention will be described in detail in conjunction with the accompanying figures, in view of the illustrative embodiments.